Moving coil electrical instrument



March30, 1954 H. J. LOVEGROVE ETAL 2,673,959

' MOVING COIL ELECTRICAL INSTRUMENT Original Filed Jan. 8, 1945 2Sheets-Sheet l wen/Zara:

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2,673,959- MOVING COIL ELECTRICAL INSTRUMENT Henry Joseph Lovegrove andMaurice Graham McBride, Enfield, England, assignors to Sangamo ElectricCompany, Springfield, 111., a corporation of Illinois Originalapplication January 8, 1945, Serial No. 571,872. Divided and thisapplication December 6, 1950, Serial No. 204,387

3 Claims. (01. 3Z4150) The present invention concerns improvements in orrelating to moving coil electrical measuring instruments and it has forits object an improved construction by which an enlarged scale range maybe obtained and in addition other advantages as will be pointed outhereafter. The present application is divisional of our copendingapplication Serial No. 571,872, filed January 18, 1945, now issued asPatent No. 2,572,639, granted October 23, 1951.

According to the invention in order to obtain a large angular scalerange the moving coil electrical measuring instrument is provided withtwo pole pieces, one consisting of an annular core which is surroundedfor a large angular range by the other pole piece, leaving a narrow airgap which is traversed by one side of the moving coil, the other side ofthe coil passing through the hole in the annular core, the annular corebeing formed of the two parts both of solid magnetic material, oneconsisting of an annular member having a gap therein to permit of thecoil being located in position and the other part being adapted to fitneatly in the core to complete the gap.

It has already been proposed to obtain in a moving coil instrument ascale extending over 270 between an inner annular member and an outermagnetic member but in this case the inner annular member was formed oflaminations which had to be fed alternately from opposite sides throughthe moving coil before the core was assembled.

An advantage of the present invention is that the core can be readilymachined either for the purpose of altering the scale or for theadaptation of the instrument for use as a ratiometer, ohmmeter or thelike.

A subsidiary feature of the invention concerns the provision of ac-shaped permanent magnet with like poles at each end and like orconsequent poles at the middle, the end poles being connected to theouter pole piece and the adjacent poles to the inner pole piece. I

According to a further subsidiary feature of the invention, the pivotsand pivot bases are mounted on the coil so avoiding the use of aseparate stafi".

According to a further subsidiary feature of the invention an electricalcircuit is provided for increasing the sensitivity of the instrumentwhen used as a ratiometer for measuring small varia tions of ratio, theelements of the circuit being made small and fitted within theinstrument casins so that the only terminals required are for nod thepower supply and the resistance to be meas- I ured.

The invention will be better understood by referring to the accompanyingdrawings in which: I Fig. 1 shows a perspective View of a single coil.

moving scale instrument.

Fig. 2 shows an end view and Fig. 3 a sectional elevation of Fig. 2.

Fig. 4 shows a perspective view of a ratiometer having two coils side byside, while in the same plane.

Referring to Figs. 1 to 3 of the drawings it will 1 be seen that themoving coil instrument consists of a C-shaped permanent magnet ll havingan outer pole piece l2 consisting of a rectangular block, the end facesof which abut on the end and having a faces of the C-shaped magnet Hcircular hole extending from side to side which is approximately equallyspaced inner central part of the C-shaped magnet so that there is a gapextending the full width of this face. The inner pole piece is formed oftwo parts I 3 and M, the part l3 it will be noted consists of an annularcore having a small gap l3b which serves to allow the coil [6 beingplaced in position. The coil it it will be noted embraces the limb ofthe annular core having one side thereof extending through the hole l3ain the core and the other side in an arcuate gap lZa between the polepieces l2 and 13. The part I4 consists of a rectangular member shaped soas to have a projection I5 for fitting in the gap l3b and serving toconnect magnetically the centre of the C-shaped permanent magnet 'H tothe annular core it both magnetically and mechanically. The pole-piece[2, the annular core l3 and the rectangular block M are all made ofsolid magnetic material so that they can be readily machined and shapedas required and furthermore a good fit can be provided between themagnetic surfaces, the reluctance of which should be as low as possible.

The C-shaped magnet is magnetised so as to have like polarity at eachend and like poles at the centre so that the direction of magnetic fluxin the arrangement illustrated is always in one direction either fromthe outer pole piece to the inner pole piece or vice versa. End platessuch as I! and I8 may be provided for securing the component partstogether, these being shaped as shown in Figs. 2 and 3. Each end platell, l8

from the two i end faces and is nearer to the face opposite the has arelatively long arcuate slot l2b therein which registers with thearcuate air gap l2a defined between the outer and inner pole pieces l2and I3. Defined within this arcuate slot 12b is a downwardly extendinghanger or tongue portion l9. This hanger portion has an axial openingl9a therei from the bottom of which extends an assembly slot l9b. Thecoil l3 embraces this hanger portion 19, and the inner side of the coilcan be assembled up into the opening [9a through the assembly slot I9b.In order that the outer pole piece I2 and inner core l3 be maintained inaccurate concentric alignment, an annular registering rib projectsoutwardly from each end of the core [3 for centering engagement in theaxial opening 19a of the hanger portion 19, and an arcuate registeringrib 2| projects outwardly from each end of the block [2 for likecentering engagement in the arcuate slot l2b of each end plate. However,it will be understood that if required either pole piece or core or bothpole piece and core could be separately located in jigs and bolted intoposition. The coil I6 is wound on a former in well-known way but in thepresent invention instead of providing a separate bearing shaft thepivot bases 22 and pivots are mounted on the coil. Furthermore thepointer 23, and balance weights 24 which are necessary owing to theeccentricity of the coil, are also secured to one of the pivot bases asshown. Compensation for temperature can be effected by inserting piecesof metal 25 whose magnetic properties vary with temperature in therecesses between the upper side of the block forming the outer polepiece and the centre of the permanent magnet.

It will of course be understood that while in the arrangement shown theannular member is coaxial with the inner cylindrical surface of theouter pole piece, this is not essential but it is desirable whenever asubstantially uniform magnetic field is required in order to obtain auniform scale. Control springs, not shown are provided on the instrumentpreferably being secured in known manner to the pivot bases on the coil.

The design of instrument described with reference to Figs. 1 to 3 lendsitself to a very convenient form of ratiometer, a description of whichwill be given with reference to Figs. 4 to 6. Like reference numbers areused for similar parts that is to say, ll represents the c-shaped magnetwith consequent poles, 12 represents the rectangular block between theend faces of the cshaped magnet, 13 represents the annular core which inthis case may be specially shaped, l4 represents the rectangularconnecting block, 16 represents the former on which the two coils arewound and 23 represents the pointer.

In the arrangement shown in Fig. 4 it will be noted that the annularcore I3 is so shaped that the gap between the annular core and the innercylindrical surface of the lateral hole extending from side to side ofthe rectangular block l2 has a varying width so as to provide a magneticfield which varies in strength round its circumference so that at alltimes the two windings on the coil arranged side by side operate infields of different strength. Currents through the coils therefore causeforces to be generated dependent upon the product of the mean strengthof the field in which the coils lie and the current flowing in thecoils. This results in the coil taking up a position where the twoforces are equal and thereby the pointer rangement shown can indicate ona scale the ratio of the two currents.

The general operation resembles that described in connection with ourBritish Patent No. 556,258 as does also the construction and arrangementof winding of the coils. In this case however there is only one side ofthe coil which works in the gap and the other works in the hole in theannular core and therefore the windings therein are not subject to anyforces created by the currents flowing in the side of the windings inthe core.

In Fig. 5 the annular core is made up of two parts 31 and 33, while thecoils 32 and 34 are arranged so as to lie in the same plane. The twoparts of the core would be slotted to permit of the coil being placed inposition and would be secured together by means of the connection withthe rectangular bar and its projecting piece for entering into the slotsof both parts of the core. As illustrated the annular core is ofcircular section as regards the outer face, that is the face facing theair gap, but is of varying widths.

In Fig. 5 it will be noted that the maximum width of the part 3| is injuxtaposition to the minimum width of the part 33 and vice versa.Consequently when one coil is in a field for substantially the wholelength of one side, the other coil is in a field for only a small partof the length of one side. There is however .a mid position when bothcoils lie in fields of equal strength and of equal length and this isthe position which the coil takes up when the currents therein are equalassuming of course the number of turns and other conditions are equal.Any variation of strength of the current in one coil with respect to theother will result in a movement of the coils until they take up aposition where the relation of the magnetic fields is inverselyproportional to the ratio of the currents. The arin Fig. 6 closelyresembles that shown in Fig. 5 as in this case one of the parts of theannular core 31 is of uniform section, while the other part 38corresponds in shape to either of the parts 3| or 33 of Fig. 5.

The construction of the coils and the operation is similar except ofcourse that the variations of ratio for which the instrument will caterare less. This arrangement will therefore give a more open scale forsmall variations of ratio of currents in the two coils.

The ratiometer as described is adapted for use as it stands with directcurrent operation only as if alternating current is applied thevariation of direction causes the device to become unstable and thepointer of the instrument will go to either one extreme position oranother, that is to say while it may function alright for one half cycleit will not function for the other half cycle and the pointer may takeup any position as it is not constrained by electromagnetic torques. Byemploying a rectifier however it is possible to make this devicefunction satisfactorily. Smoothing is not necessary and a transformercan adjust the voltage desirable working limits if the source of supplyis too high or too low.

A no-volt indicator can conveniently be incorporated in the ratiometerdescribed with reference to Figs. 4 or 5 or 6 and the moving element maybe of soft iron with a restoring spring to return it to a definiteposition when energy is not connected. Alternatively a magnet steel ofhigh coercive value may be employed for the moving element which ispolarised so that when energy is not connected to the instrument themoving element assumes one position and is held definitely in thisposition by leakage flux from the main permanent magnet system of theinstrument. When energy is connected to the instrument, flux is set upby the coil which overcomes the leakage flux and causes the element totake up a new position indicating that the ratiometer is functioningcorrectly.

In connection with ratiometers due to the large degree of movement carehas to be taken in the design of the ligaments. Conveniently these aremade very thin and of rectangular cross section. The length of theligament is preferably about double the distance between the pivot ofthe coil and the stationary abutment where it is secured. The ligamentis mounted at the moving end approximately on a line passing through thepivoting center, and at the fixed end on a line approximately atright-angles to the line.

joining the pivoting center to the fixed abutment.

What we claim is:

1. In a moving coil electric measuring instrument of the classdescribed, the combination of a single C-shaped permanent magnet havingconsequent magnetic poles of one polarity at its center and terminalmagnetic poles of the other polarity at its ends, stationary outer andinner pole pieces joined to the terminal end poles and to the consequentcentral poles respectively of said permanent magnet, said stationaryouter pole piece having a substantially cylindrical opening extendingtherethrough, said stationary inner pole piece comprising twosubstantially cylindrical hollow core sections disposed in axialalignment in the cylindrical opening of said outer pole piece and havingtheir adjacent ends axially separated, the cylindrical outer surfaces ofboth hollow core sections being spaced substantially uniform radialdistances from the inner surface of said cylindrical opening in theouter pole piece to define radially extending arcuate magnetic gapsbetween each core section and said outer pole piece of substantiallyuniform radial dimension, said two inner core sections being soconnected to said permanent magnet as to be of the same magneticpolarity as the consequent central poles of said permanent magnet, andsaid outer pole piece being so connected to the terminal end poles ofsaid permanent magnet as to be of the opposite magnetic polarity, apivotally mounted moving system comprising two coils axially separatedand embracing each of said axially separated hollow core sections, saidcoils having their inner side legs passing through the axial holes intheir respective hollow core sections and having their outer side legspassing through said magnetic gaps, one of said hollow core sectionscomprising a spirally sloping end surface having one direction of pitchand the other hollow core section comprising a spirally sloping endsurface having the opposite direction of pitch, whereby said two coresections present respectively increasing and decreasing widths ofperipheral pole surfaces to their respective air gaps and coils forcausing said coils to take up a rotary position corresponding to theratio of the currents flowing in them.

2. In a ratiometer of the class described, the combination of anintegral substantially C- shaped permanent magnet comprising a onepiecesolid upper portion having consequent magnetic poles in the center ofsaid solid upper portion, and comprising spaced lower side arms integralwith said one-piece solid upper portion and. terminating insubstantially vertical inwardly. facing polar end surfaces, an outerpole piece.

outer pole piece having a substantially circular hole extendinglongitudinally therethrough and opening upwardly through the top of saidouter pole piece, an inner pole piece comprising two substantiallycylindrical hollow core sections disposed in axial alignment in thecylindrical open-.

ing of said outer pole piece and having their adjacent ends axiallyseparated, the cylindrical outersurfaces of both hollow core sectionsbeing spaced substantially uniform radial distances from the innersurface of said cylindrical openingv in the outer pole piece to definearcuate magnetic gaps between each of said two core sections and saidouter pole piece of substantially uniform radial dimension, a pivotallymounted moving system comprising two coils axially separated andembracing each of said axially separated hollow core sections, saidcoils having their inner side legs passing the axial holes in theirrespective hollow core sections and having their outer side legs passingthrough said magnetic gaps, each of said hollow core sections comprisingan integral substantially C-shaped portion comprising a solid bottomportion and a longitudinal slot in its upper portion opening upwardlytherefrom to permit the inner side leg of its respective coil to beinserted into the axial bore of said core section, a block of magneticmaterial comprising a, tongue fitting into the longitudinal slots inboth of said hollow core sections after the coils have been assembledtherein, said tongue extending substantially the entire axial length ofboth hollow core sections, the top side of said block pressing upwardlyagainst the under side of the solid upper portion of said permanentmagnet in registration with said consequent poles whereby both of saidhollow core sections have the same magnetic polarity as said consequentpoles, one of said hollow core sections having a spirally sloping endsurface whereby said core section presents a varying width of peripheralpole surface to its respective air gap so that the coil moving in saidgap will move in a field of varying strength in the rotary motion ofsaid moving system.

3. In a ratiometer of the class described, the combination of anintegral substantially C- shaped permanent magnet comprising a onepiecesolid upper portion having consequent magnetic poles in the center ofsaid upper portion, and downwardly and inwardly turned end portionsterminating in parallel, substantially vertical inwardly facing endsurfaces having terminal magnetic poles therein, said permanent magnetbeing of single one-piece construction extending from one of saidinwardly facing polar end surfaces entirely around the structure to theother inwardly facing polar end surface. whereby the permanent magnet ischaracterized by a relatively long length of magnet structure within agiven outer dimension and relatively short length of spacing between theopposite inwardly facing polar end surfaces, an outer pole piece in theform of an approximately rectangular block of soft iron havingsubstantially parallel, vertical outwardly facing end surfaces abuttingagainst the inwardly facing polar end surfaces of said permanent.magnet, said outer pole piece havin a. substantially circular holeextending longitudinally 'therethrough and opening upwardly through thetop of said outer pole piece, an inner pole piece comprising twosubstantially cylindrical hollow core sections disposed in axialalignment in the substantially circular hole of said outer pole pieceand having their adjacent ends axially separated, the cylindrical outersurfaces of both hollow core sections being spaced substantially uniformradial distances from the inner surface of said circular hole in theouter pole piece to define radially extending arcuate magnetic gapsbetween said two core sections and said outer pole piece ofsubstantially uniform radial dimension, a pivotally mounted movingsystem comprising two coils axially separated and embracing each of saidaxially separated hollow core sections, said coils having their innerside legs passing through the axial holes in their respective hollowcore sections and having their outer side legs passing through saidmagnetic gaps, said two hollow core sections each having a longitudinalslot therein to permit the inner side leg of its respective coil to beinserted into the axial bore of its respective hollow core section, anda block of magnetic material joined to the consequent poles of saidpermanent magnet and comprising a tongue extending into the longitudinalslots in both of said hollow core sections whereby said hollow coresections have the same magnetic polarity as said consequent poles, oneof said hollow core sections comprising a spirally sloping end surfacehaving one direction of pitch and the other hollow core sectioncomprising a spirally sloping end surface having the opposite directionof pitch, whereby said two core sections present respectively increasingand decreasing widths of their peripheral pole surfaces to theirrespective air gaps and coils for causing said coils to take up a rotaryposition corresponding to the ratio of the currents flowing in them.

HENRY JOSEPH LOVEGROVE. MAURICE GRAHAM MCBRIDE.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,171,907 Trent Feb. 15, 1916 1,918,023 Faus July 11, 19332,218,376 Corson Oct. 15, 1940 2,346,683 Hickok Apr. 18, 1944 2,367,950Lenehan Jan. 23, 1945 FOREIGN PATENTS Number Country Date 483,539 GreatBritain Apr. 21, 1938

